Ceramic paper generally refers to high temperature resistant, insulating inorganic sheet material having a thickness of up to 1/4 of an inch and predominantly comprising ceramic fibers. Such materials are referred to as "paper" because of their resemblance to wood pulp papers and because they can be rolled on conventional papermaking machines. Such paper is used for a wide variety of household and industrial applications. Recently, the automotive industry has become interested in ceramic paper for use in the inflator filter units of automotive airbags.
In particular, the automotive airbag manufacturers are attempting to make more efficient use of the propellant or generant, which is very expensive, by designing inflators which burn hotter. The hotter burning inflators result in optimizing reaction completion which, in turn, allows the airbag manufacturer to use less propellant or generant in the inflator, and thus, reduce cost.
However, such a very high output inflator is useless without a compatible ceramic paper filter material which is strong enough, flexible enough, and porous enough to withstand the high output blast to which it is subjected. Furthermore, any such ceramic paper filter material must be free of organics so that outgassing does not add to the internal combustion pressure and temperature or to the toxicity of the gas.
More specifically, an inflator filter unit of ceramic paper desirably should have a minimum standard for tensile strength of at least about 195 gm/cm (500 grams/inch), preferably 315 gm/cm (800 grams/inch) and minimum low pressure/high flow rate of about 1.84 cubic meters per minute (65 cubic feet per minute). Also, any such filter unit should be organic-free (0% Loss On Ignition) and exhibit no dustiness.
Heretofore, ceramic papers relied principally on latex, polyvinyl acetate and phenolic resin type binders for achieving required tensile strength properties. However, as noted hereinabove, upon exposure to high temperatures, these organic binders would frequently burn out, causing outgassing which would result in a loss of tensile strength for the paper and the release of combustion products into the atmosphere, and when used in airbag inflators, would increase the temperature and pressure to variable levels, a result which is not desired. Air bag inflator filters today employ at least some organic binder material, typically, about three percent (3%). However, even this rather low amount of organic binder may create variations in temperature and pressure.
In an effort to overcome this problem of outgassing, one alternative which has been tried was the production of an inorganic sheet without organic binders. However, virtually all attempts to prepare inorganic ceramic papers have so far proven unsuccessful in one way or another. Some attempts yielded very weak products, e.g., burn grade papers, while others resulted in formulating papers with limited flexibility and rollability. For example, Barber, Jr. U.S. Pat. No. 5,053,107 provides high temperature resistant, insulating inorganic ceramic papers for use in high temperature environments which do not include any organic binders. Instead, the ceramic paper produced in this patent is held together by fiberglass fibers such as E glass, ranging in length from 100 .mu.m to 0.75 inches in place of the organic binders. In fact, the fiberglass fibers are actually melted to the ceramic fibers. Notably, however, these ceramic papers become brittle at temperatures greater than 1400.degree. F. and are otherwise not easily rolled. Moreover, this ceramic paper is not nearly as flexible as is required for use in inflator units.
Thus, while substantial effort has been made to prepare organics-free papers with high tensile strength, good flexibility, porosity and flow properties, retaining these properties in the same inorganic ceramic paper at commercially acceptable levels has not previously been achieved.
Accordingly, a need exists for a strong, porous, flexible and rollable ceramic paper which is free of all organic binders. Such ceramic paper would be very beneficial to air bag manufacturers in the development of an organics-free inflator filter unit.